This invention relates to a three-phase voltage protective circuit which continuously monitors the power phases of a polyphase power source and actuates a contactor to close or open and to provide power to a three-phase load when the three phases are in proper voltage and phase relation to one another, but to cut off power to the load if the integrity of any of the three phases becomes degraded.
Polyphase protective circuits, as currently available, are complex and expensive devices which rely heavily on digital controls and elaborate circuitry. These devices can cost over $600 each, and represent a sizeable expense in electrical power design for large three-phase induction motors.
Larger polyphase induction motors have a three phase contactor (which is essentially a three-pole relay) interposed between the three phase power line and the motor. The contactor is actuated to start the motor. A protective three-phase monitor is generally connected to the contactor and has probes connected to the three phases of AC power in advance of the contactor.
The phase relations of the three power phases and their voltage levels are continuously monitored. In the event of a phase reversal or a loss of power in one of the three power conductors, the protective monitor will open the contactor and de-energize the motor. If the phase angle between the three power conductors varies from 120 degrees, the monitor will signal an alarm condition and then open the contactor to shut the motor off if phase dissymmetry persists or exceeds some threshold for a specified period of time. Also, the voltage is monitored, and the contactor is held open if a high or low voltage condition occurs.
These devices sense voltage and phase condition only upstream of the contactor. If a voltage or phase condition occurs due to a contactor malfunction, the problem will not be detected and the motor will not be protected.